It should work on a non ADC monitor. (The old motherboards does not support the ADC supplying power through the card) But for driving a VGA or DVI it should work just fine
Replacing the ATI 128 on my G4/400 with a Radeon 8500 helped Quake III and even UT even if is supposed to be CPU limited. FAKK 2 became playable in 1024x768
A 9700 is an overkill as is the 8500 and even the 9000 so I would go for the cheapest aviable. perhaps that 2mx card or the Radeon AGP? A company calleed Wagner Media or something like that had a small stock of 2mx cards a while back and sold them for 30 dollars
For non games I have not seen any difference between the ATI 128 and the R8500